The recovery of hydrocarbons from subterranean zones relies on the process of drilling wellbores. The process includes drilling equipment situated at surface, and a drill string extending from the surface equipment to a below-surface formation or subterranean zone of interest. The terminal end of the drill string includes a drill bit for drilling (or extending) the wellbore. The process also involves a drilling fluid system, which in most cases uses a drilling mud that is pumped through the inside of piping of the drill string to cool and lubricate the drill bit. The drilling mud exits the drill string via the drill bit and returns to surface carrying rock cuttings produced by the drilling operation. The drilling mud also helps control bottom hole pressure and prevent hydrocarbon influx from the formation into the wellbore, which can potentially cause a blow out at surface.
Directional drilling is the process of steering a well from vertical to intersect a target endpoint or follow a prescribed path. At the terminal end of the drill string is a bottom-hole-assembly (“BHA”) which comprises 1) the drill bit; 2) a steerable downhole motor of a rotary steerable system; 3) sensors of survey equipment used in logging-while-drilling (“LWD”) and/or measurement-while-drilling (“MWD”) to evaluate downhole conditions as drilling progresses; 4) means for telemetering data to surface; and 5) other control equipment such as stabilizers or heavy weight drill collars. The BHA is conveyed into the wellbore by a string of metallic tubulars (i.e. drill pipe). MWD equipment is used to provide downhole sensor and status information to surface while drilling in a near real-time mode. This information is used by a rig crew to make decisions about controlling and steering the well to optimize the drilling speed and trajectory based on numerous factors, including lease boundaries, existing wells, formation properties, and hydrocarbon size and location. The rig crew can make intentional deviations from the planned wellbore path as necessary based on the information gathered from the downhole sensors during the drilling process. The ability to obtain real-time MWD data allows for a relatively more economical and more efficient drilling operation.
One type of downhole MWD telemetry known as mud pulse (MP) telemetry involves creating pressure waves (“pulses”) in the drilling mud circulating through the drill string. Drilling mud is circulated from surface to downhole using positive displacement pumps. The resulting flow rate of drilling mud is typically constant. The pressure pulses are achieved by changing the flow area and/or path of the drilling fluid as it passes the MWD tool in a timed, coded sequence, thereby creating pressure differentials in the drilling fluid. The pressure differentials or pulses may be either negative pulses or positive pulses. Valves that open and close a bypass stream from inside the drill pipe to the wellbore annulus create a negative pressure pulse. Valves that use a controlled restriction within the circulating drilling mud stream create a positive pressure pulse. Pulse frequency is typically governed by pulse generator motor speed changes. The pulse generator motor requires electrical connectivity with the other elements of the MWD probe.
In MP telemetry systems, as well as in other downhole tools, the pulse generating motor driveline system is subjected to extreme pressure differentials of about 20,000 psi between the external and internal aspects of the tool. To accommodate this large pressure differential, drilling mud is allowed access to areas of the tool which are positioned on one side of a compensation mechanism. Pressure is equalized on the other side of the pressure compensation mechanism within the tool using clean, non-drilling fluid such as hydraulic fluid or silicon oil. Various systems have been used to provide pressure compensation including metallic bellows, rubber compensation membranes, and piston compensations with springs. Given the large temperature differentials from surface to downhole, especially in colder drilling climates, there is a high chance of temperature related failures for MWD tool components, in particular rubber membranes used for pressure compensation.
During MP telemetry the operation of a mud pulser can cause wear and breakdown of the annular seal which fluidly seals the rotating driveshaft of the mud pulser from the external drilling mud. The motor of the mud pulser is typically enveloped in lubricating oil which is contained in the pulser housing by the annular seal. With time, oil may leak out and drilling mud may leak in through the worn seal and the seal must be replaced before any substantial amount of mud leaks in. Drilling mud within the motor housing is detrimental to the operation of the motor, bearings and gearbox, and these components will typically be destroyed if a substantial amount of drilling mud enters the motor housing.